In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, the spent nuclear fuel (hereinafter, “SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, SNF is first placed in a hermetically sealed canister that creates a confinement boundary about the SNF. The loaded canister is then transported and stored in a large cylindrical container called a cask. Generally, a transfer cask is used to transport SNF from location to location while a storage cask is used to store SNF for a determined period of time.
One type of storage cask is a ventilated vertical overpack (“VVO”). A VVO is a massive structure made principally from steel and concrete and is used to store a canister loaded with SNF. In using a VVO to store SNF, a canister loaded with SNF is placed in the cavity of the body of the VVO. Because the SNF is still producing a considerable amount of heat when it is placed in the VVO for storage, it is necessary that the cavity is vented so that this heat energy has a means to escape from the VVO cavity. It is also imperative that the VVO provide adequate radiation shielding and that the SNF not be directly exposed to the external environment. Thus, a need exists for a VVO system for the storage of radioactive materials that provides enhanced ventilation, reduces the likelihood of radiation exposure, and provides sufficient radiation blockage of both gamma and neutron radiation emanating from the high level radioactive waste.